The present invention relates to styrene-based polymers and a process for production thereof. More particularly, it is concerned with novel styrene-based polymers which have a branching nucleus containing a sulfur atom and further which have specified sulfur content, molecular weight and molecular weight distribution, and a process for efficiently producing the styrene-based polymers. 2. Description of Related Art
The impact resistance of styrene-based polymers can be improved by increasing the molecular weight thereof. In general, however, if the molecular weight is increased, fluidity will drop and, therefore, residual strain, etc. are increased in molding and no good molding can be obtained.
In order to increase the fluidity of the polymer, it is considered to raise the molding temperature, or decrease the molecular weight, or add an internal lubricant. Raising the molding temperature, however, is accompanied by a problem that the resulting molding is subject to thermal deterioration and colored in yellow. If the molecular weight is decreased, mechanical properties such as impact resistance and so forth are reduced, and if the internal lubricant is added, heat resistance is decreased.
Under such circumstances, it has been desired to develop a technique to improve the impact resistance of styrene-based polymers without deteriorating the fluidity thereof.
It is known that so-called styrene-based star polymers are obtained by anion-polymerizing styrene-based polymers by the use of a polymerization initiator, e.g., metallic lithium (see, for example, J. Polym. Sci. PART, A 3, 681-696 (1965)). Styrene-based star polymers produced by the anionic polymerization, however, have such a narrow molecular weight distribution that the weight average molecular weight/number average molecular weight (Mw/Mn) is about 1:1, and further have a disadvantage in that if the molecular weight is increased, moldability will be seriously reduced.
A method of producing styrene-based star polymers by radical polymerizing styrene-based monomers is described in Makcromol. Chem., 178, 1427 (1977). In this publication, however, only the mechanism of the reaction is studied, and the conversions of the polymers obtained are at most 25%. That is, there is found no description concerning styrene-based star polymers obtained by radical polymerizing at a conversion in excess of 25%. There is also no description concerning the physical properties and moldability of the styrene-based star polymers obtained. The present inventors' investigation has revealed that the styrene-based star polymers obtained by the method disclosed in the above publication do not have satisfactory physical properties and moldability.